Antimony as a Programmable Element in Integrated Nanophotonics

The use of nonlinear elements with memory as photonic computing components has seen a huge surge in interest in recent years with the rise of artificial intelligence and machine learning. A key component is the nonlinear element itself. A class of materials known as phase change materials has been extensively used to demonstrate the viability of such computing. However, such materials continue to have relatively slow switching speeds, and issues with cyclability related to phase segregation of phase change alloys. Here, using antimony (Sb) thin films with thicknesses less than 5 nm we demonstrate reversible, ultrafast switching on an integrated photonic platform with retention time of tens of seconds. We use subpicosecond pulses, the shortest used to switch such elements, to program seven distinct memory levels. This portends their use in ultrafast nanophotonic applications ranging from nanophotonic beam steerers to nanoscale integrated elements for photonic computing.


S2.Switching Speed Measurements 1
To calculate the switching speed of Sb on waveguide, we perform time resolved switching 2 experiments using a 4 port device as shown in Figure S2 (a). Probe signal output is 3 connected to a 125 MHz photodetector to record the change in transmission on sending a 4 Pump pulse. A single write pulse is used to switch the material from crystalline state to 5 amorphous state. Due to low signal to noise ratio , we average the change for a set of 10 6 reading and find a rise time (from 10% of change to 90%) of 2ns , which corresponds to 7 operation speed of up to 500 MHz ( Figure S2 (b)). The experiments are repeated for 8 different pump power to achieve different memory levels using single sub picosecond pulse 9 of energy P1-P4, with P1>P4. 10 Furthermore, the switching speed of Sb was compared with other well-known phase 11 change material using the pump-probe technique. The switching speeds of Sb, AIST and GST Figure S 2 (a) Optical image of 4 port device used to measure the switching speed. Phase change material is deposited on the optical crossing. Using one of the ports as input for pulse and other as probe, time resolved switching characteristics is obtained. (b) Time resolved switching dynamics of Sb, resulting in a rise time of 2ns for a single sub picosecond pulse, on switching from crystalline to amorphous state. (c) Experimental comparison of switching speed of Sb is with other known phase change materials like GST and AIST using a single sub picosecond pulse for amorphisation. are on same timescales as shown in Figure S2(c). However, as shown in our previous work 23 , 1 a very short femtosecond pulse is enough to amorphise Sb, therefore the current switching 2 speed for Sb is limited only by photonic system rather than material itself. 3

S3.Sb Length Dependence on contrast 4
Experimental results showing the effect of length of Sb on the loss in amorphous and 5 crystalline state. Different lengths of Sb were deposited on a waveguide and the 6 transmission of the as deposited Sb on waveguide was noted. The experiments were 7 repeated after annealing the samples on a hot plate at 230 ℃ for 5 minutes to completely 8 crystallize Sb. Increasing the length of Sb results in a higher absorption and hence a larger 9 contrast ( Figure S3). 10

S4.Effect of Capping Layer 11
Further to our experiments with uncapped antimony thin films. We studied the 12 performance of our device with an additional 10nm capping layer of Indium tin oxide (ITO). 13 Due to an increased absorption of light due to the capping layer, a 4 µm long device is 14 enough to get a 10% change in contrast as compared to 10µm long uncapped device ( Figure  15 S4(a)). Further we investigated Raman spectra of both capped and uncapped crystalline 16 antimony, after annealing in air at 230 ℃. No antimony oxide (Sb2O3) peaks at 191cm -1 and 1 255 cm -1 were observed in our samples ( Figure S4 (b)). 2  To calculate the energy we use a Pyroelectric sensor from 'Ophir', which has an energy 1 measurement resolution of few Nano joule to hundreds of Nano joule, with a maximum 2 repetition rate of 15 kHz. To accurately measure the energy of a single pulse we send train of 3 240-800 pulses and corresponding energies are recorded. This is used to estimate energy of 4 single pulse. Each train of pulse is sent 100 times. The average and the standard deviation 5 obtained is used to report the switching energy and the error associated. 6 S6. Sub-millisecond Readout 7

S5.Pulse Energy calculations
We characterise the stability and repeatability of achieving the binary memory levels in sub 8 millisecond range. 10 µm Sb device is amorphized using single fs pulse and subsequently 9 crystallized using 100 low energy fs pulses after 100 µs. The change in transmission is 10 recorded using an oscilloscope Figure S6(a). The switching is repeated multiple time and 11 shows a repeatability with less than 2% variation, multiple plots are stitched together and 12 presented in Figure S6 (b). 13

S7. Multilevel Readout stability 14
We characterise the stability and repeatability of achieving the 4 intermediate memory levels 15 in millisecond range. 10 µm Sb device is amorphized using single fs pulse and subsequently 16 crystallized using 100 low energy fs pulses. The change in transmission is recorded using an 17